As he did with Perseus, so Ovid will strangely bend the tale of Medea in Book 7, and again he'll seem to shift the emphasis from its major features to others elements that might strike us as less important.
Since he relies on our knowledge of the back-story -- indeed, of Euripides' play as well -- here's the brief version of how the Golden Fleece came to hang in the kingdom of Aeetes:
In Greek mythology, Phrixus or Frixos (Greek: Φρίξος, Phrixos) or Phryxus was the son of Athamas, king of Boiotia, and Nephele (a goddess of clouds). His twin sister Helle and he were hated by their stepmother, Ino. Ino hatched a devious plot to get rid of the twins, roasting all of Boeotia's crop seeds so they would not grow. The local farmers, frightened of famine, asked a nearby oracle for assistance. Ino bribed the men sent to the oracle to lie and tell the others that the oracle required the sacrifice of Phrixus and Helle. Before they were killed, though, Phrixus and Helle were rescued by a flying, or swimming, ram with golden wool sent by Nephele, their natural mother; their starting point is variously recorded as Halos in Thessaly and Orchomenus in Boeotia. During their flight Helle swooned, fell off the ram and drowned in the Dardanelles, renamed the Hellespont (sea of Helle), but Phrixus survived all the way to Colchis, where King Aeëtes, the son of the sun god Helios, took him in and treated him kindly, giving Phrixus his daughter, Chalciope, in marriage. In gratitude, Phrixus sacrificed the ram to Zeus and gave the king the golden fleece of the ram, which Aeëtes hung in a tree in the holy grove of Ares in his kingdom, guarded by a dragon that never slept.